square/retrofit · error · IllegalStateException

Response must be parameterized as Response<Foo> or Response<

Error message

Response must be parameterized as Response<Foo> or Response<? extends Foo>

What it means

Thrown by Java8CallAdapterFactory when the CompletableFuture is parameterized but its argument is raw `Response` (e.g. `CompletableFuture<Response>`). The factory needs `Response<T>` to extract T for deserialization; the raw inner type fails the check at Java8CallAdapterFactory.java:79.

Source

Thrown at retrofit-adapters/java8/src/main/java/retrofit2/adapter/java8/Java8CallAdapterFactory.java:80

      Type returnType, Annotation[] annotations, Retrofit retrofit) {
    if (getRawType(returnType) != CompletableFuture.class) {
      return null;
    }
    if (!(returnType instanceof ParameterizedType)) {
      throw new IllegalStateException(
          "CompletableFuture return type must be parameterized"
              + " as CompletableFuture<Foo> or CompletableFuture<? extends Foo>");
    }
    Type innerType = getParameterUpperBound(0, (ParameterizedType) returnType);

    if (getRawType(innerType) != Response.class) {
      // Generic type is not Response<T>. Use it for body-only adapter.
      return new BodyCallAdapter<>(innerType);
    }

    // Generic type is Response<T>. Extract T and create the Response version of the adapter.
    if (!(innerType instanceof ParameterizedType)) {
      throw new IllegalStateException(
          "Response must be parameterized" + " as Response<Foo> or Response<? extends Foo>");
    }
    Type responseType = getParameterUpperBound(0, (ParameterizedType) innerType);
    return new ResponseCallAdapter<>(responseType);
  }

  private static final class BodyCallAdapter<R> implements CallAdapter<R, CompletableFuture<R>> {
    private final Type responseType;

    BodyCallAdapter(Type responseType) {
      this.responseType = responseType;
    }

    @Override
    public Type responseType() {
      return responseType;
    }

View on GitHub (pinned to d0b112dad0)

Solutions

1. Parameterize the inner Response: `CompletableFuture<Response<User>> getUser();`.
2. Or drop the Response wrapper entirely: `CompletableFuture<User> getUser();`.
3. Prefer the built-in CompletableFuture support over the deprecated factory.

Example fix

// before
@GET("users/{id}")
CompletableFuture<Response> getUser(@Path("id") String id);

// after
@GET("users/{id}")
CompletableFuture<Response<User>> getUser(@Path("id") String id);

// Check for raw retrofit2.Response as a direct type argument of CompletableFuture.
for (Method m : MyService.class.getDeclaredMethods()) {
    Type rt = m.getGenericReturnType();
    if (rt instanceof ParameterizedType) {
        Type arg = ((ParameterizedType) rt).getActualTypeArguments()[0];
        if (arg == retrofit2.Response.class) {
            throw new IllegalStateException(m + " uses raw Response inside CompletableFuture.");
        }
    }
}

// Fail loud in a wiring test:
try {
    retrofit.create(MyService.class).getUser("1");
} catch (IllegalStateException e) {
    if (e.getMessage().contains("Response must be parameterized")) {
        throw new IllegalStateException("Fix raw Response<> inside CompletableFuture in MyService.", e);
    }
    throw e;
}

When it happens

Trigger: A service method returns `CompletableFuture<Response>` (raw Response) instead of `CompletableFuture<Response<Foo>>`.

Common situations: Switching a body return to a Response-wrapped return and dropping the inner type argument; autocompleting Response without generics; refactoring without re-running the wiring test.

Related errors

• CompletableFuture return type must be parameterized as Compl
• Response must be parameterized as Response<Foo> or Response<
• Response must be parameterized as Response<Foo> or Response<
• Response must be parameterized as Response<Foo> or Response<
• ListenableFuture return type must be parameterized as Listen

AI-assisted analysis of square/retrofit@d0b112dad0 (2026-08-04). Data as JSON: /data/errors/c3adff7287a1108c.json. Report an issue: GitHub.